An unsaturated carboxylate having a vinyl group is useful as a starting material for oxidation-curable type coating resins, unsaturated polyester resins, photo-curable resins, and silane-coupling agents, and further as a crosslinking agent for acrylic rubbers. In these uses, allylmethacrylates are widely employed at the present time.
However double bond in the allylmethacrylates is unstable, resulting in that there has been a problem of gelling in preparation of polymers of the allylmethacrylates and in radical copolymerization with a variety of acrylic monomers. Furthermore, allyl group is rigid because of few carbon numbers, resulting in that resins obtained therefrom do not have sufficient flexibility. Still further, a reaction velocity in hydrosilylation is slow in the case of preparation of silane coupling agents, and unreacted double bonds often remain in the reaction system.
In the meantime, hitherto, there have been used a variety of resin curing systems in coatings, adhesives, and photo-curable resins.
In the resin systems, there is widely employed a curing system in which there is employed a (meth)acrylic resin having epoxy groups as side chains obtained by (co)polymerization of a (meth)acrylate having epoxy group, owing to exceeding usefulness.
As the (meth)acrylates having epoxy group to be employed in the curing system, there are well known glycidyl methacrylate, beta-methylglycidyl (meth)acrylate, 3,4-epoxycyclohexylmethyl(meth)acrylate. For example, Japanese Patent Publication (Kokai) No. 45577/1990 describes a coating composition in which there is employed a curing reaction between glycidyl methacrylate and compounds having carboxylic groups. Furthermore, Japanese Patent Publication (Kokai) No. 73825/1990 describes a coating composition in which there is employed a cationic curing reaction of an alicyclic epoxy(meth)acrylate such as 3,4-epoxycyclohexylmethyl(meth)acrylate with compounds having silanol groups.
However, as glycidyl methacrylate has an exceedingly high reactivity with the compounds having carboxylic group, in the case when it is employed as a one-liquid type coating composition, a pot-life is short, resulting in causing a problem of gelation. Furthermore, glycidyl group is slow in curing reaction velocity in the case of the use of cationic catalysts.
Accordingly, it is substantially difficult to employ glycidyl methacrylate in the curing system described in the Kokai No. 73825/1990.
Still further, alicyclic epoxy groups in the alicyclic epoxy(meth)acrylate such as 3,4-epoxycyclohexylmethyl(meth)acrylate has a high reactivity in a cationic curing, accordingly, in the case when the alicyclic epoxy(meth)acrylate is employed in a coating system containing compounds having carboxylic groups or in the presence of catalysts having a relatively strong acidity such as phosphoric acid, oxalic acid, p-toluene sulfonic acid, there is a problem that a pot-life is short because a cationic polymerization is caused by hydroxyl groups in the system which act as an initiator, resulting in that there is difficult the selection of curing catalysts exhibiting an appropriate pot-life.
On the other hand, beta-methylglycidyl (meth)acrylate has a mild reactivity with compounds having carboxylic groups or in cationic curing, accordingly, although a pot-life can be lengthened in one-liquid type coatings, there has been a problem that a curing reaction time of period is lengthened or higher curing temperatures require, resulting in that a selection range in curing conditions is narrow.
Furthermore, glycidyl (meth)acrylate and beta-methylglycidyl (meth)acrylate contain chlorine even after purification because these are prepared from epichlorohydrin and beta-methylepichlorohydrin, which are starting materials, respectively, resulting in that it is not able to prevent corrosion in substrates and yellowing in coatings during a curing reaction for baking.
For the purpose of solving the above-mentioned disadvantages, as a result of an intensive investigation, the inventors of this invention have now found that it is possible to solve the various problems by resins in which a (meth)acrylate having an alkenyl group and an epoxy(meth)acrylate are (co)polymerized, respectively.
The (meth)acrylate having an alkenyl group of the present invention can provide a (meth)acrylic resin having alkenyl group-containing side chains by the (co)polymerization in the presence or absence of a variety of polymerizable monomers. The (meth)acrylic resin can be employed as resins for curable reactions in coatings, photo-curable resin, and adhesives, etc. As the (meth)acrylic resin contains double bonds as side chains differently from allylmethacrylate copolymers, it can provide a cured article having excellent flexibility. Furthermore the (meth)acrylic resin optionally contains an inner double bond, it can provide a cured article having an improved crosslinking density.
An epoxy(meth)acrylate of the present invention can provide a (meth)acrylic resin having epoxy group-containing side chains obtained by the (co)polymerization in the presence or absence of a variety of polymerizable monomers. The (meth)acrylic resin has a well-balanced property between a pot-life and reactivity in curing, and has an anti-corrosion property to substrates and anti-yellowing property.
The (meth)acrylic resin having epoxy group-containing side chains of the present invention can provide a thermosetting resin composition which can be employed as a coating composition.
In the case when the thermosetting resin composition of the present invention is employed as a coating composition, it can provide a coating layer having flexibility because epoxy groups which are crosslinking points are situated at a position separated from the main chain of the (meth)acrylic resin.
In the meantime, for more than ten years, powder coating compositions have been widely used in many fields, because, e.g., of the following excellent properties:
(a) it does not contain any solvents, therefore, it has an advantage of presenting less physiological hazards and environmental pollution hazards, and avoiding the risk of fire; PA1 (b) it requires only reasonable costs, because as mentioned above, solvents are not used, and because excess parts of a powder coating composition which is not fixed onto the substrate to be coated at the time of application can be recovered completely; PA1 (c) it has a capability of being used to form thick coating layers of up to 100 microns, which cannot be achieved with paints or varnishes having solvents; PA1 (d) its coated and cured layer does not tend to soften even when exposed to an elevated temperature atmosphere; and PA1 (e) it has a characteristic of better adhesion to metal substrates.
An acid-curing type powder coating by use of a polybasic acid or a carboxyl-terminated polyester resin having at least 2 carboxylic groups in a molecule has been widely used because of various kinds of excellent properties, such as ductility of the coating layer, surface hardness of the coating layer, etc., in addition to the above-described (a) to (e).
They can be attained by adjusting the molecular weight and by appropriate selection of a combination of a polycarboxylic acid(s) and a polyhydric alcohol(s), said polyester resin can be readily prepared by an esterification reaction between (n+1) mole of a polycarboxylic acid(s) and n mole of a polyhydric alcohol(s).
A polyester resin having carboxyl groups in terminal positions usually reacts with an epoxy resin to form a cured coating layer as described hereinafter.
Namely, carboxyl groups in terminal positions and epoxy groups react and cure by heating at the presence of catalyst, resulting in formation of tough coating layer.
So-called epi-bis type epoxy resins produced by a reaction between bisphenol A and epichlorohydrin, novolak type epoxy resins produced by a reaction between a novolak phenol resin and epichlorohydrin and the like have been used in the above-described curing reaction with a polyester resin. However, the above-described epoxy resin cannot give sufficient resistance to heat and good outdoor durability to the coated layer made from a corresponding powder coating composition.
Furthermore, a powder coating in which a (meth)acrylic resin having epoxy groups are mixed with a curing agent having carboxylic groups is widely used because of excellent weatherability and gloss.
Hitherto, glycidyl methacrylate has been widely used as the (meth)acrylic resin having epoxy groups. However, the epoxy group in glycidyl methacrylate has an exceedingly high reactivity with the compounds having carboxylic group as described hereinabove.
Accordingly, in the case when a (meth)acrylic resin containing glycidyl methacrylate is kneaded with a curing agent and pigments, etc. to prepare a powder coating composition, there were disadvantages that the powder coating composition is not sufficiently kneaded because of the reaction between epoxy groups and carboxylic groups, and fluidity in melting for baking is insufficient, resulting in that coating layer therefrom does not exhibit an excellent smoothness.
Furthermore, in the case when a carboxyl-terminated polyester resin is employed as the curing agent having carboxylic groups, it is poor in compatibility with the (meth)acrylic resin containing glycidyl methacrylate, resulting in that the smoothness in coating layer therefrom is considerably deteriorated and occasionally pinholes are caused.
In recent years, triglycidyl isocyanurate has been used because a powder coating composition prepared with it can form a layer having excellent resistance to heat, good outdoor durability, and also smoothness on the surface of coating layer.
However, triglycidyl isocyanurate has a human toxicity.
Furthermore, there has increasingly been required highly advantageous properties according to the extension of uses of powder coating compositions in various fields.
The (meth)acrylic resin having epoxy group-containing side chains of the present invention can also provide a powder coating composition having a milder reactivity than epoxy(meth)acrylates having an alicyclic epoxy group, and a faster reactivity than epoxy(meth)acrylates having glycidyl group or beta-methylglycidyl group.